Silicon-based integrated photonics allows for the formation of compact lightwave circuits and is compatible with silicon electronics and standard complementary metal-oxide-semiconductor (CMOS) fabrication methods. The high refractive index contrast between the silicon core and the surrounding silica (SiO2) enables the propagation of highly confined optical modes, which allows scaling integrated photonic waveguides down to submicron level. A result of this high refractive index contrast is that silicon-based integrated photonic waveguides exhibit large modal structural birefringence between the two orthogonal transverse electric (TE) and transverse magnetic (TM) fundamental modes of the guided light. Hence, in a conventional 450-nm-wide by 220-nm-high silicon waveguide, the TE and TM modes generally have effective refractive indices that differ substantially, for example around 2.35 and 1.74 respectively. Due to its greater tolerance to bending losses, the TE mode is preferred in most practical applications.
An existing approach to couple light into a silicon photonic chip involves an inverted taper. For example, the width of a 450-nm-wide and 220-nm-high waveguide (single-mode operation at 1550 nm) can be longitudinally tapered down to a tip having a width of about 150 nm. As light propagates toward the tip of the taper, the mode size increases while the effective mode index decreases. For example, in the case of a TE mode propagating in a 220-nm-high silicon waveguide whose width is longitudinally tapered from 450 nm down to 150 nm, the modal index gradually decreases from 2.35 to 1.46 while the mode size increases up to about 2 to 3 μm. It is therefore possible to use such an inverted taper to couple light to another waveguide having similar lateral dimensions such as a lensed fiber or a very-high numerical-aperture fiber. However, using this approach for coupling to more common waveguides such as, for example, optical fibers with mode field diameters (MFDs) ranging from about 5 μm to 10 μm, leads to high coupling losses in practice. In a silicon-on-insulator (SOI) platform, the silicon waveguide sits on a 2- to 3-μm-thick buried oxide (BOX) layer, which itself sits on a silicon substrate having a thickness of several hundreds microns. If the mode expands outside of the inverted taper to increase to diameters greater than a few microns, it will tend to leak toward the silicon substrate which will be detrimental to the coupling efficiency to the optical fiber.
Recently, subwavelength gratings have been proposed to add flexibility in the design of waveguiding structures, such as described in a paper by Cheben [P. Cheben et al., “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers”, Optics Letters, Vol. 35, Issue 15, pp. 2526-2528 (2010)]. When alternating the nature of the materials (typically silicon and silica) along the propagation axis of a waveguiding structure following a pattern having a period which is less than half the wavelength of the guided light, the overall waveguide behaves as a uniform material having an intermediate refractive index whose value lies between those of the two alternating materials. This can provide a way to engineer the refractive index of a waveguide to tailor its optical mode. However, using subwavelength gratings alone may not be sufficient to increase the mode size beyond what can be achieved with an inverted taper although it may provide a more practical way of obtaining the same result.
Accordingly, various challenges exist in the development of spot-size converters for photonic integrated circuits that can achieve efficient optical mode conversion and coupling between two dissimilar waveguides.